Known in the art is a flexible pipe whose design makes it possible to transmit working media under increased pressure and at the same time to receive an axial tensile force (French Pat. No. 2,142,764). Said flexible pipe comprises a rubber supporting pipe onto which pipe several layers of metal braid are wound in spiral. The lowest layer and the uppermost layer are wound with a shift in the winding angles of from 6.degree. to 80.degree.. Metal wires in all the layers are subject to tension and allow the flexible pipe cross-section to be preserved at a slight bending thereof under conditions of simultaneous influence of gage internal pressure and a slight axial tensile force.
However, such a flexible pipe cannot be applied in the case of effect of distributed or local external load, e.g. under the influence of gage external pressure. Under the effect of compression in the radial direction, the cross-section of the flexible pipe loses its stability due to the fact that the metal wires constituting the braid cannot resist this effect.
Also known in the art is a flexible pipe which is intended to receive, without deformation, apart from gage internal pressure and an axial tensile force, gage external pressure (USSR Inventor's Certificate No. 668,625). In accordance with the above disclosure, this flexible pipe comprises an inner supporting pipe constructed from an elastic material onto which pipe a flat strip from a rigid material (e.g. metal) is wound. The above strip forms a cylindrical power casing with an interlayer from an elastic material being put thereupon. Two layers of shaped rods are wound in crossed directions onto said interlayer. The cross-section of each rod has a maximum size in the radial direction. Said rods are wound symmetrically at angles of up to 40.degree..
Undoubtedly, the presence of the rods wound at such angles will allow the flexible pipe to receive an axial tensile force and gage internal pressure without fracture. However, if such a flexible pipe is placed into a medium whose pressure exceeds the pressure within the flexible pipe, the deformation of the flexible pipe becomes possible even at the pressure drop of 4 to 5 kg/cm.sup.2, said deformation being accompanied by the loss in stability of its cross-section. This is due to the fact that the pressure of the external medium is freely transmitted through the gaps between the shaped rods (especially under tension) to the interlayer made from an elastic material and, correspondingly, to the power frame formed by the flat strip. The rigidity of this frame in the radial direction is low and is in no way reinforced by the shaped rods which are by themselves subject only to tension. Thus, the power frame, after having lost its stability, will be locally pressed into the flexible pipe. Simultaneously, the turns of the flat strip will separate, and the tightness of the flexible pipe will be certainly upset. Naturally, the above shortcomings will take place only at differentials in internal and external pressure of more than 4 to 5 kg/cm.sup.2.
To our opinion, the most successful is a construction of a flexible pipe developed by the company "Coflexip S.A." (see advertising brochure "Coflexip Flexible Pipe for the Offshore Industry" by this company, published in 1979). The above flexible pipe comprises an inner supporting pipe constructed from an elastic material, a shaped strip having a Z-shaped cross-section and wound in a spiral onto said pipe. The Z-shaped cross-section of the strip is formed by two stepwise conjugated parallel flanges. The upper and the lower flanges are conjugated therebetween by means of a leg. Each flange is constructed in the form of an angle and rests by the edge of this angle on the opposite flange of the next turn of the strip. Thickness of each flange is substantially less (5 to 10 times) than the height of the strip profile. The above design is substantially one of modifications of the invention of a S-shaped profile. Thus, said angles of adjacent turns of a strip are put into engagement and limit the possibility of extending the power frame formed by the strip, in the axial direction. An interlayer of an elastic material is superimposed onto the power frame formed by a shaped strip. On the interlayer, there is provided at least one pair of reinforcing layers formed by groups of power filaments, said groups being parallel to one another, and wound substantially in a symmetrical position. The power filaments are provided in the form of steel wires and are wound at angles of 40.degree. to 60.degree. to the axis of the flexible pipe. The flexible pipe is also provided with an external protective shell made from an elastic material.
An obvious advantage of the above described construction of the flexible pipe consists in that such a design makes it possible to receive three types of loads: an axial tensile force, gage internal pressure, and gage external pressure. A sufficiently good perception of the gage external pressure is promoted by the fact that the strip forming the power frame is made in the shaped form, and all the structure of the flexible pipe is hermetically sealed by the external protective shell. While possessing the above advantages, the flexible pipe can maintain its flexibility.
Nevertheless, in the manufacture of such a flexible pipe it is necessary to utilize such expensive and high quality materials as alloy steels, titanium etc. Such a need is caused by the following considerations: a comparatively small thickness of flanges is required to provide for reliable engagement between the angles thereof because with the above mentioned angles of winding the power filaments, the latter do not ensure the complete perception of the total axial tensile force. Therefore, to avoid distortion of uniformity of the power frame (i.e. to eliminate the possibility of formation of through gaps between the strip turns under tension), the presence of engaging angles is necessary. For this reason, and also due to a rather complex technology of strip shaping, reinforcement of said strip by increasing the thickness of the flanges is impracticable. Moreover, the upper flange of each turn of the strip rests on the lower flange of the adjacent turn with a comparatively narrow edge of the angle. Consequently, the material from which the shaped strip is constructed, must possess high physico-mechanical properties, including high hardness among them. It will be understood that in order to reinforce the power frame, the power filaments of the reinforcing layers are also to be constructed from a high-strength steel wire. It should be also noted that the strength of the power frame receiving a portion of the axial tensile force is determined essentially by the strength of the weakest joint between two adjacent turns of the strip. The above consideration imposes especially high demands upon the production process of the shaped strip and the frame.